Practical Screw Thread Information & Tolerances

[Gordon Clarke]

Hi Gordon,

The Info about Designation of screw threads of Whitworth form is to all. I already know you are familiar with screw threads.

Let's both hope there are designers here in the Cove

Giving information on a drawing takes only a few seconds but if insufficient, can give the machine operator nightmares or at minimum extra work. For example writing "just" H8 or h7 (or 6H or 6g if a metric thread )on a drawing often means that the tolerance must be looked up - probably every single time the drawing hits the workshop.
I know one guy that, when machinig a thread, looked at tolerance H6 (hole) and confused it with 6H (internal thread). There's quite a difference

 

[Wayne]

...do you know of any thread type other than Whitworth that uses/has a flank angle of 55 degrees?

In all cases where the thread angle is identified as 55 degrees, in my experience the thread is called Whitworth. Even in the BSPP and BSPT styles.

I have seen the thread designated, and we designate the thread when offering a gage quote, with a leading "W", but can not confirm if this is authorized by some standard. It is in keeping with the common thread designation conventions within the ISO community (i.e.: G1/4; R1/4; Rc1/4; M4; MJ4; Tr5; Rd20; Pg7).

 

[Wayne]

Giving information on a drawing takes only a few seconds but if insufficient, can give the machine operator nightmares or at minimum extra work.

A good design and a well documented drawing take time which costs money. A poorly documented drawing takes time and money and time and money and time and money everytime it is used.

According to the Pennsylvania Dutch: "The hurrieder you go the behinder you get."

 

[Gordon Clarke]

In all cases where the thread angle is identified as 55 degrees, in my experience the thread is called Whitworth. Even in the BSPP and BSPT styles.

I have seen the thread designated, and we designate the thread when offering a gage quote, with a leading "W", but can not confirm if this is authorized by some standard. It is in keeping with the common thread designation conventions within the ISO community (i.e.: G1/4; R1/4; Rc1/4; M4; MJ4; Tr5; Rd20; Pg7).

Every thread designation you give is easily recognised by myself and everyone else I know that works with screw threads. There isn't any risk of "mistaken" identity
I see G used much more often than BSPS for Whitworth straight pipe threads whereas R is used for Whitworth tapered threads BSPT.


Wayne will know this but to those that don't then:
Rp = internal parallel thread for fits made on the thread
Rc = internal tapered threads for fits made on the thread
R = external tapered thread fo fits made on the thread.
i.e. An external tapered thread is always tapered.
Tapered Whitworth tapered threads : ISO 7-1 and
Parallel Whitworth pipe threads : ISO 228-1

 

[Wayne]

...An external tapered thread is always tapered

Gordon; sorry to disagree, but there is an external R-series thread which is parallel. See: BS-21; Appendix C. Special parallel external threads for gas appliances where pressure-tight seals are made on machined faces. This appendix defines the Rs thread.

Thus the British Standard Pipe list revised:
ISO-7 & BS-21 Rc = internal tapered threads for fits made on the thread
ISO-7 & BS-21 R = external tapered thread for fits made on the thread.
ISO-7 & BS-21 Rp = internal parallel thread for fits made on the thread
BS-21 Rs = external parallel thread for fits made on machined face.
ISO-228 G = internal or external threads for fits where seal is not on the thread.

 

[Gordon Clarke]

Gordon; sorry to disagree, but there is an external R-series thread which is parallel. See: BS-21; Appendix C. Special parallel external threads for gas appliances where pressure-tight seals are made on machined faces. This appendix defines the Rs thread.

Thus the British Standard Pipe list revised:
ISO-7 & BS-21 Rc = internal tapered threads for fits made on the thread
ISO-7 & BS-21 R = external tapered thread for fits made on the thread.
ISO-7 & BS-21 Rp = internal parallel thread for fits made on the thread
BS-21 Rs = external parallel thread for fits made on machined face.
ISO-228 G = internal or external threads for fits where seal is not on the thread.

It isn't chisseled in stone that I'm always right so if you say so I'll just nod and accept it - for now
I looked up BS21 and came up with:
https://www.piccadillyprecision.co.uk/products_details.asp?cid=6
It was the first one I Googled and I have nothing to do with the company.

Glancing at it I come up with the question that
ISO 7-1 is Pipe threads where pressure-tight joints are made on the threads
and
ISO 228-1 is Pipe threads where pressure-tight joints are not made on the threads
What's the difference then between "your" parallel external R series thread and the G series? - apart from the fact that they are "special".

Im honestly trying to figure it out and not trying to mess with your head

 

[stefanhg]

Hi Gordon, Wayne,

BS 21:1985 is replaced by BS EN 10226-1:2004, BS EN 10226-2:2005 and BS EN 10226-3:2005.

Parts 1 and 2 reflect the two thread jointing systems in regular use
in Europe and Worldwide - Part 1 details taper external and parallel internal pipe threads, and Part 2 details taper external and taper internal pipe threads. Part 3 provide requirements for the gauging of pipe threads
conforming to Parts 1 and 2.

Because of the different assembly techniques used for the taper / parallel and taper / taper systems, it is recommended that mixing of components having parallel internal threads and taper internal threads is avoided in
the same piping system.

What's the difference then between "your" parallel external R series thread and the G series?

Profile and nominal diameters are the same, tolerances and allowances are different.


Thread; Rs1 ; G1
TPI ;11 ; 11
pitch ;2,309 ; 2,309
Depth of thread ;1,479 ; 1,479
Major d. ;33,249 ;33,249
Max. ;33,038 ;33,249
Min. ;32,708 ;32,889
Pitch d. ;31,770 ;31,770
Max. ;31,559 ;31,770
Min. ;31,379 ;31,590 (A), 31,410 (B)
Minor d. ;30,291 ;30,291
Max. ;30,080 ;30,291

 

[Wayne]

Glancing at it I come up with the question that
ISO 7-1 is Pipe threads where pressure-tight joints are made on the threads
and
ISO 228-1 is Pipe threads where pressure-tight joints are not made on the threads
What's the difference then between "your" parallel external R series thread and the G series? - apart from the fact that they are "special".

I'm honestly trying to figure it out and not trying to mess with your head

First, let me state that the Rs-thread is very rarely used. I have not done a pitch diameter comparison between it and the G-series, the usage is so low that it has not warranted my time to make the comparative study.

Next, the actual difference between the two parallel thread types is only discovered when comparing the pitch diameters. I have made a brief look at these and have found that they are very close in size.

Finally, the stated difference is that the G-series is designed to screw together and not bind at any point in the assembly until the thread is screwed tight to another surface; where the Rp-thread is designed to be mated with a tapered R-thread and is designed to crash into the tapered R-thread and cause a seal at the crash point.

 

[Wayne]

BS 21:1985 is replaced by BS EN 10226-1:2004, BS EN 10226-2:2005 and BS EN 10226-3:2005.

Stefanhg;
This statement was true. BS 21 was rescinded and replaced with BS EN 10226 which mimics ISO 7, but there was an outcry from users of BS 21 who did not want to change to the ISO-like gage system, and BS 21 was reinstated.

 

[Gordon Clarke]

Hi Gordon, Wayne,

BS 21:1985 is replaced by BS EN 10226-1:2004, BS EN 10226-2:2005 and BS EN 10226-3:2005.

Parts 1 and 2 reflect the two thread jointing systems in regular use
in Europe and Worldwide - Part 1 details taper external and parallel internal pipe threads, and Part 2 details taper external and taper internal pipe threads. Part 3 provide requirements for the gauging of pipe threads
conforming to Parts 1 and 2.

Because of the different assembly techniques used for the taper / parallel and taper / taper systems, it is recommended that mixing of components having parallel internal threads and taper internal threads is avoided in
the same piping system.

Profile and nominal diameters are the same, tolerances and allowances are different.

Thanks Stefan,
From what you write then it seems as if common sense prevails at last within bureaucracy. Putting all three types (thanks Wayne for making me aware of the mystery third type ) of Whitworth form pipe threads as "one" standard seems to good to be true.
An old dog (me) can learn new tricks